Metal cleansing



Patented Nov. 4, 1941 METAL CLEANSING James N. Ostrofsky, Baltimore, Md., asslgnor to Rustlcss Iron and Steel Corporation, Baltimore,

No Drawing.

7 Claims.

. My invention relates to the cleansing or descaling of metallic products, especially the prod-. ucts fashioned of stainless iron and steel, and more particularly to a conditioning or treating of the scale thereon to permit ready removal.

One of the objects of my invention is the provision of a simple, eflicient and reliable manner of treating scale formed on metallic articles in treating operations quickly and efiiciently without in any way damaging the surface of these products.

Other objects in part will be obvious and in part pointed out hereinafter.

The invention accordingly consists in the various operational steps and in the combination of steps and the relation of one or more of the same to one or more of the others, as described herein, the scope of the application of which is indicated in the following claims.

As conducive to a clearer understanding of certain features of my invention it may be noted at this point that certain metals, such as, the stainless irons and steels acquire a close, tough, adherent scale as a result of heating. This scale must be removed from the metal to give a desired bright finished surface or in order to permit further working. In heretofore known processes of cleansing metals, numerous practical difliculties are encountered.

Considering more particularly the stainless irons and steels, these alloy irons and steels contain chromium in the amount of from about 10 per cent to 35 per cent or more. In addition, they may contain one or more of a number of ingredients, such as, manganese, silicon, aluminum, copper, nickel, molybdenum, cobalt, tungsten, or titanium, vanadium, columbium, tantalum, or sulphur, phosphorous or tellurium, for special purposes. Because of the large amount of chromium present in these steels, and also because ofcertain others of the refractory metals where present as modifying ingredients, an exceptionally dense scale appears on the surface of a product during heating and certain working operations.

' It seems that this scale largely consists of var- Application December 30, 1937, Serial No. 182,631

ions oxides of chromium in solid solution with certain oxides of iron. The relative proportions of the iron and chromium oxide content of this solid solution in a measure depends upon the particular grade of stainless steel. The toughness, color, density and other physical charac-- teristics of this scale differ somewhat with the analysis of the stainless steel. In all grades,

however, it is found that the scale is adherent,

a in aqueous solution with dense and exceedingly dimcult to remove.

In working a billet into wire, for example, the metal is conducted through a number of successive operations. It is customary to reheat the metal between each of these operations. During this heating, a scale forms on the product. This scale must be removed before coldrolling or cold-drawing the metal in order that a high-grade finished product may be realized.

In heretofore known processes of removing scale from metallic articles of the character indicated, a strong acid pickle ordinarily is relied upon. Certain inhibitors are used in the pickling solution to minimize the attack on the base metal of the product. In other processes a combination of acidic and basic treatments are used. In still others, a combination of these treatments certain electrolytic treatments are resorted to. In one or two molten alkali is used either with or without electrical treatment. I

Theoretically there is as much reason to expect decomposition of scale by cathodic treatment in molten caustic as by anodic treatment. In the former case a reduction to metal would be expected. In the latter case oxidation to sodium compounds. Pratically it is found that where stainless alloys are given cathodic treatment, a black or dark gray coating remains which is removed only with difficulty by common pickling solutions and not at all by straight nitric acid. This is in spite of the fact that the bulk of the original scale is actually reduced. I do not know the nature of this coating-whether it is non-reducible matter, partly decomposed scale baked on, or compounds plated on from the bath itself.

On the other hand, when stainless iron and steel is given an anodic treatment alone in caustic, some definite action does occur as can be seen by a change in color and texture of the scale. An orange cast appears rather uniformly over the surface. But the scale so treated is very insoluble in common pickling solutions and shows little solubility in nitric acid. Furthermore, the appearance of scale only anodicly steel, stain the surface of the product so that further treatment is of great dimculty.

One of the objects of my invention, therefore, is the cleansing or de-scaling of metal products, particularly stainless iron and steel products, in an efilcient and thoroughly reliable manner, permitting a subsequent wash or cleansing in dilute acid to give a bright finished surface untamished and unpitted.

I have found that, if the scale is first opened up by a cathodic treatment of sufficient duration in fused caustic to thoroughly break up the scale, the subsequent anodic treatment in the caustic would be much more efficient, penetrating through the entire scale. The resultant scale or surface product is still insoluble in pickling solutions and in straight nitric acid. A further treatment as cathode in the fused caustic results in a decomposed scale or surface product which is vigorously acted on by nitric acid and completely removed in a few minutes, leaving a white clean surface. The appearance before treatment with nitric acid seems no different than that of scale treated only as cathode, but the great difference in solubility in nitric acid is remarkable. I do not know if this result occurs because of the burning away of non-reducible compounds during the anodic treatment or because of a change in composition but it is a fact that subsequent treatment as cathode produces a scale entirely different in acid solubility than can be produced by cathodic treatment alone. Nitric acid is, of course, an ideal reagent for use with stainless iron and steel because of its lack of action on these steels and because it can be readily handled in stainless iron and steel equipment.

This scale or surface product, resulting from my three-step treatmentor the interruption of straight cathodic treatment by anodic treatment-is also readily soluble in the common (oxidizing) pickling solutions such as nitric-hydrofluoric, nitric-sulphuric, or nitric-hydrochloric mixtures of various strengths. When pickled finishes, such as are typical of these solutions, are desired, the decomposed scale can be removed entirely with these agents. Where it is difiicult to affect complete decomposition of scale in caustic-because of, for instance, (1) the shape of the work, (2) inability to use ample current density because of heat effects, or (3) insuillcient throwing power for the work in handit is advisable to use (oxidizing) pickling solutions, rather than straight nitric acid-so that action on both decomposed scale, as well as any traces of undecomposed scale will be obtained. It is also more soluble in non-oxidizing pickling solutions such as inhibited 10% sulphuric acid, but these solutions are only used with stainless irons and steels as a pre-treatment because of the undesirable dirty gray color characteristic of pickle finishes from them. They can be used if followed by brightening dips.

The nature of the scale found on stainless iron and steel varies greatly with the grade, previous operations, whether hot-rolled, forged, temperature of heat treatment and furnace atmosphere.

For this reason, it is dimcult to prescribe definite conditions of operation as to current density and duration of each cycle of treatment in the caustic.

These are best determined by actual experiment with the work at hand. On light scale on cold drawn wire, for instance, 25 amperes per square foot will sometimes be found sufficient, although this is probably near the low limit of current density that would be used. Generally with hotrolled scale, especially on the iron-chromiumnickel alloys, I prefer to use to 200 amperes per square foot, or even higher. The only restriction as to an upper limit is the current carrying capacity of the work to be cleaned and the usual restrictions of economy. A shorter time will be required, the higher the current density, and conversely the same effect can be produced by lower current densities, if employed for longer ,treated annealed hot-rolled wire, composition being .08% carbon, 17.50% chromium, 8.75% nickel, for two minutes as cathode, three minutes as anode, and two minutes as cathode, all at a current density of amperes per square foot, and obtained a. decomposed scale which was completely removed by three minutes immersion in nitric acid at room temperature. The strength of the nitric acid was 1 volume, 40 Baum commercial nitric acid to 3 volumes of water. Again I have treated cold-drawn and annealed .185" diameter wire, composition being 0.11% carbon, .30% sulphur, 12.75% chromium, .30% nickel, for one-half minute as cathode, then three minutes as anode, then one-half minute as cathode, at 50 amperes per square foot current density and obtained a decomposed scale which was readily removed in 10% volume nitric acid in two minutes at room temperature.

I believe lower temperatures in the molten range.of sodium hydroxide are to be preferred because of the cooling effect on the current carrying wire. Heavier current densities can then be employed without bringing the wire to damaging temperatures. On the other hand, if very low temperatures are employed, drag-out of caustic may become excessive. A balanc must be struck here but these are obviously matters of simple operating economy and do not eflect the disclosure, as I have found the same broad benefits obtained from my three-step treatmentcathodic, anodic, cathodicat all temperatures from just above the melting point of caustic up to 1100 F. and above.

In general each step of the three-step processcathodic, anodic, cathodic-should be of about the same duration. Thus, if wire will not clean at a certain current density at exposures of 1-l1 minutes, the time of each step should be increased for instance to 333 minutes respectively. In a few cases a somewhat longer anodic treatment has seemed to be of benefit as 1-31 minutes respectively on free-machining grades, but in general this is not so. I wish to emphasize. as a matter of instruction and not in a limiting sense, that each step performs a definite function and that, in general, the time required to perform this work is about the same for each step.

While as illustrative of the practice of my invention, treatment of rustless iron and steel products are particularly considered, it will be understood that the practice is applicable to other chromium alloy irons or steels. Likewise, while a treatment comprising three successive electrolytic steps is described, it will be understood that additional steps may be introduced when a particularly dense scale is encountered. Moreover, while it is indicated that the products treated are immediately dipped into the acid pickling solution, this is found to be somewhat wasteful of acid, because of the considerable amounts of acid neutralized by the alkali dragged out of the bath with the work. Savings of this expensive acid are had by first rinsing the products in running water or dipping in a tank of water. More economical operation is had, however, by dipping the products in a weak solution of an inexpensive acid, such as sulphuric. This step affords a saving in the expensive pickling solution, into which the product is then immersed. Moreover, the salt formed does not give of! the objectionable fumes which characterizes the dipping of the hot product into water.

As many possible embodiments may be made of my invention, and as many changes may be made in the embodiment hereinbefore set forth. it will be understood that all matter described herein is to be taken as illustrative, and not in a limiting sense.

I claim:

1. In removing surface scale from chromium alloy iron products, the art which includes, subjecting said products to a first cathodic treatment in fused caustic for at least one-half minute andat a current density of at least amperes per square foot in order to open up the scale, followed by an anodic treatment in said caustic of about the same duration and current density to penetrate the scale and change its character, and then by a second cathodic treatment in said caustic for a like period and current density in order to decompose the scale permitting its subsequent ready removal.

2. In removing surface scale from stainless iron products, the art which includes, subjecting said products to electrolytic treatment in a bath of fused caustic at a current density of at least about 100 amperes per square foot, first rendering said products cathodic for at least about a minute in order to open up the scale, then anodic for a like period of time to penetrate the scale and change its character, and finally cathodic to decompose the scale to permit its subsequent ready removal.

3. In removing surface scale from stainless iron products, the art which includes, subjecting said products to a first cathodic treatment in fused sodium hydroxide for at least one-half minute at a current density of at least 25 amperes per square foot in order to open up the scale, followed by an anodic treatment for a similar period of time and at a similar current denisity in order to penetrate the scale and change its character, and then by a second cathodic treatment in said fused hydroxide under like conditions of time and current density to decompose the scale to permit its ready removal.

4. In removing surface scale from stainless iron products containing at least 10% chromium, the art which includes, subjecting said products to an electrolytic bath in a fused caustic for several minutes time and at a current density of at least 25 amperes per square foot, first rendering said products cathodic, then anodic and finally cathodic in said bath each for at least one-half minute in order to open up, penetrate and finally loosen the scale and then pickling said products in an aqueous solution of nitric acid to clean the same.

5. In removing surface scale from stainless iron products'containing from 10 per cent to 35 per cent chromium, the art which includes, subjecting said products to electrolytic treatment in a bath of fused caustic, first rendering the said products cathodic, then anodic and finally cathodic in said bath each for intervals of several minutes and at current densities of at least 25 amperes per square foot in order to open up, penetrate, loosen and decompose the scale on the same; treating said products with an acid to neutralize the caustic removed from the bath; and then pickling the products to remove the scale therefrom.

6. In removing surface scale from stainless iron products containing about 12 per cent chromium, the art which includes subjecting said products to electrolytic treatment in fused caustic at current densities of at least 25 amperes per square foot, rendering said products cathodic, anodic and finally cathodic. each for at least one-half minute, in order to loosen, change and decompose the scale to permit its subsequent ready removal.

'I. In removing surface scale from stainless iron products containing about 18% chromium and 8% nickel. the art which includes subjecting the products to electrolytic treatment in fused caustic at current densities between about and 200 amperes per square foot, rendering said products cathodic, anodic, and finally cathodic, each for at least two minutes, in order to loosen, change and decompose the scale to permit its subsequent ready removal.

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